As a substrate for mounting a silicon power semiconductor element, there is known as shown in Patent Document 1, for example, a circuit substrate which comprises a ceramic substrate formed of an aluminum nitride sintered member, a silicon nitride sintered member or the like, and a metal plate made of copper, a copper alloy or the like, bonded with the ceramic substrate by brazing. In Patent Document 1, the metal plate after the bonding is made to have a Vickers hardness of 60 or more, in order to prevent a crack from occurring in the ceramic substrate due to a difference in thermal expansion between the substrate and the metal plate.
Power semiconductor elements such as switching elements (IGBT, MOSFET, etc.,) and rectifier elements, that are used for power semiconductor devices, such as inverters, are required to reduce their power losses. Thus, there have recently been developed power semiconductor elements using wide bandgap semiconductors, such as silicon carbide (SiC) and gallium nitride, etc. In the case of the wide bandgap semiconductor element, since this element has itself a higher thermal resistance, it is operable at a high temperature with a large current; however, in order to exert such a feature, it is required that the power semiconductor element and the circuit substrate be tightly bonded together.
Using a solder or a brazing material for this bonding as a bonding material, requires a higher temperature for bonding than the operation temperature of the element, and is, therefore, difficult to be applied to the power semiconductor device which uses a wide bandgap semiconductor and is supposed to operate at a high temperature. As a result, metal-nanoparticles, which can be sintered at a temperature lower than the melting point of the corresponding bulk metal and are tolerable up to the melting point of the bulk metal after sintering, is becoming adopted as a bonding material for high temperature operation use. For example, in Patent Document 2, there is disclosed that metal nanoparticles of gold, silver, copper or the like, with a size of 50 nm or less are used when a SiC semiconductor is bonded with an electrode on a substrate.